Charging of batteries with an inductive power supply is well-known. Inductive charging of batteries for electric automobiles as well as the charging of small electric appliance batteries such as those for toothbrushes has met some amount of success. Because inductive charging does not require a physical connection between the battery and the charger, the charging is considerably more convenient. However, not all batteries are easily charged inductively. Lithium-ion batteries (Li-Ion) are one such type of battery.
Recharging Li-Ion batteries is not as straightforward as that of other batteries. Li-Ion batteries are unable to absorb an overcharge. If a constant current is applied to a fully charged Li-Ion battery, metallic lithium plating may develop which could lead to failure of the battery. Thus, care should be taken not to overcharge the battery.
Conversely, charging a Li-Ion battery to full capacity presents some difficulty. The maximum voltage of a Li-Ion battery can be attained relatively quickly during recharging by applying a constant current to the battery. However, when the Li-Ion battery reaches a maximum voltage, the Li-Ion battery may not be fully charged. Without further charging, the battery will only be approximately 65% charged. If a constant current is continually applied to the battery after the battery has reached its maximum voltage, then the battery could be overcharged, which could lead to premature battery failure.
Conventional battery chargers have been developed to fully charge a Li-Ion battery. Generally, the battery charger uses a constant current, constant voltage schema to charge the battery. A discharged battery is first charged at a constant current level in the range of 0.1C to 1C amperes, where C is the battery capacity in amp-hours, until the battery reaches the desired voltage of about 4.2 volts. At this point, the battery charger switches to a constant voltage mode, providing the sufficient power to maintain the battery at this final voltage while providing additional charging to the battery.
The charging profile for a typical Li-Ion battery is shown in FIG. 1. A constant current is applied for a predetermined period. During this phase, the charging of the Li-Ion battery is generally constant. For a typical battery, this phase lasts somewhat less than one hour. The Li-Ion battery eventually exhibits a constant voltage near a preferred voltage prior to attaining a full charge. A constant voltage is then applied to the Li-Ion battery. After approximately an hour of charging with a constant voltage, the battery has typically attained its maximum charge.
If the charging of a Li-Ion battery does not follow the charging profile shown in FIG. 1, then there is a risk that the battery will not be fully charged or that the charging will damage the battery.
The charging of a Li-Ion battery is further complicated because the battery is often not fully discharged before charging. If some residual charge remains on the battery, then optimal charging may require some amount of constant current charging followed by constant voltage charging, or, alternatively, the optimal charging may require only constant voltage charging. For better performance, the battery charger should provide a mechanism for compensating for the charge state of the battery.
Charging Li-Ion batteries is especially problematic where inductive charging is used. In an inductive battery charger, a primary coil located in the charger provides power to an inductive secondary located in the battery. The voltage across the secondary is then rectified and applied to the battery to recharge the battery. There is no direct physical connection between the battery and the battery charger. Because there is no physical connection between the battery and the battery charger, information regarding the state of the battery is not readily available to the battery charger.
At the same time, portable devices need to be lightweight. Thus, complex circuitry to monitor the charge state of the battery and relay that information to the inductive charger increases the cost, size and weight of the portable device.
An inductive system capable of charging a battery having a unique charge cycle while using a relatively simple circuit directly coupled to the battery is highly desirable.